Gcse Chemistry/Biology Joint Investigation the Decomposition of H2o2 Using Catalase In Yeast As a Catalyst

How the Experiment is "fair"

Table of variables

Factor

Why it affects the reaction

Diagram (if applicable)

How it is controlled

Amount of Yeast

The more catalyst (yeast) there is the quicker the hydrogen peroxide decomposes because there is a higher concentration and there will be more collisions.

Each time I will only use0.1g of yeast.

Volume of H2O2

If there is more H2O2 then the reaction will be able to go on longer because as enzymes do not get used up. Also the collisions would happen more often because there is a higher concentration of the substrate.

Each time I will only use 10cm³ of H2O2.

Stirring

If you stir the solution the reaction would happen more quickly due to the catalase colliding with the substrate more often.

I will not stir the solution.

Time measure

The longer you time the more oxygen is going to be given off until all the H2O2 has decomposed.

I will time for 3 minutes and take a result after every 20 seconds.

Temperature of the water bath

The hotter the water bath is the quicker the reaction will be because the substrate and enzymes will be moving more quickly and thus will collide more often. This will only happen up to a certain point (until the enzyme becomes denatured)

This is the variable so I will control this as said in the method.

Prediction

I predict that as the temperature rises so will the rate of the decomposition of the H2O2. I think this will happen because in the yeast is an enzyme called catalase. This is an enzyme, which is a biological catalyst. All reactions need activation energy (the energy required to break the bonds of the substance in the reaction) to take place. A catalyst lowers the activation energy therefore making the reaction happen quicker because the bonds break more easily. As you increase the heat, the particles vibrate more vigorously which makes the bonds even easier to break. The way the catalyst speeds up the break down of the H2O2 is that it collides with the hydrogen peroxide molecules. This is stated in the collision theory "Chemical reactions occur when particles of the reactants collide. They must collide with a certain minimum energy, called the activation energy."

If you look at catalase as an enzyme rather than a chemical catalyst then the temperature will still increase the rate of the reaction. This is because enzymes are a Biological catalyst. An enzyme is not a surface for the reaction to take place on as with a chemical catalyst but it breaks down the substrate with a lock and key mechanism. This theory says that the enzyme acts as the lock and every time the substrate fits into the lock the substrate is broken down i.e.

In Mackean's it states "A 10°C rise will double the rate of reaction". So if we get 10cm³ of oxygen produced after 20 seconds at 20°C then we should get 20cm³ of oxygen produced after 20 seconds at 30°C. I also predict that at about 50°C the enzymes will start to become denatured which means that the key (the substrate) will not fit into the active site anymore i.e.

I believe that this will happen because in Mackean's it states "Above 50°C the enzymes, being proteins, are denatured and stop working." This is correct about biological catalysts (enzymes) but not about manmade catalysts e.g. magnesium(iv)oxide which do not use the lock and key mechanism so do not become denatured.

Results

My Results are as follows:

Time (sec)

O2 Produced (cm³)

20°C

30°C

40°C

50°C

60°C

70°C

st

2nd

Av

st

2nd

Av

st

2nd

Av

st

2nd

Av

st

2nd

Av

st

2nd

Av

20

3

5

4

5

5

5

5

5

5

5

5

5

5

5

5

5

5

5

40

8

8

8

1

1

1

5

7

6

2

4

3

9

7

8

5

7

6

60

0

2

1

8

6

7

24

28

26

5

7

6

5

3

4

1

3

2

80

4

6

5

26

24

25

35

37

36

21

23

22

9

21

20

5

5

5

00

9

21

20

34

30

32

40

44

42

28

30

29

23

23

23

8

8

8

20

24

24

24

39

33

36

46

50

48

33

35

34

28

28

28

22

20

21

40

28

26

27

45

41

43

52

54

53

36

38

37

30

32

31

24

26

25

60

31

33

32

50

44

47

56

56

56

40

44

42

34

34

34

26

28

27

80

33

33

33

53

49

51

58

60

59

44

46

45

38

36

37

29

31

30

From this table I can make a graph: (see next sheet)

From this graph I can draw another graph: (see sheet after next sheet)

Analysis

From the rate of reaction graph you can see that raising the temperature increases the rate of reaction up until 40°C which is the crest of the curve and is therefore the optimum temperature for the enzyme to work. After this crest the rate of reaction decreases, which implies that the enzymes have begun to denature. Also on the first graph you can see that the initial part of the reaction was the quickest. This is because the solution is at its hottest at the beginning and therefore there would be more collisions because the molecules are moving faster. Another reason this could happen is because at the start there is the largest amount of H2O2. In terms of the lock and key theory (the lock being the enzyme and the key being the substrate) there are more keys to fit into the locks, which means that the collisions will happen more often. Then later in the reaction when the H2O2 has been broken down there are less keys to fit into the locks and there will therefore be less collisions. But if you are taking the catalase as a catalyst then the reaction starts off quick and then slows down because there are more H2O2 molecules for the catalase to lower the activation energy for. The reaction slows down due to the fact that there are not as many collisions due to there being less H2O2 molecules.

I think that the reason for temperature increasing the rate of reaction is because it supplies the reaction with more energy. When the reaction has more energy more collisions take place because the molecules are moving more quickly and therefore increases the rate of reaction. This is basically what I predicted. However my prediction about 10°C rise in temperature doubles the rate of reaction was wrong. This is most probably due to the fact that our results were flawed and the experiment was not 100% accurate. I will state the problems with my method in my evaluation.

My results rise when the temperature increases but then start to decrease when the temperature is hot enough to denature the enzymes. This proves my prediction was right and it forms the conclusion that temperature affects the rate of reaction by increasing it up to a certain point (in this case about 40°C) and then decreasing it after this point.

Evaluation

The results we recorded showed the general trend of what I expected which was that heating increases the rate of reaction and heating too much denatures the enzyme. However there were results which were a little bit iffy and there is lots of room for improvement of accuracy. The result for 30°C was a little bit below what I expected, the result for 50°C was also a bit too low and the result for 70°C was too high compared to what was expected. There are many reasons why this could have happened but the main reason this happened is because we couldn't keep the hydrogen peroxide at the right temperature throughout the 3 minutes. So when we thought we were performing the experiment at 30° we were actually doing it at about 25°C for most of it which would be why the result was too low. One way to stop this happening is to keep the water at a constant temperature by heating it with an electric heater. This would increase the accuracy of the experiment because the temperature will not slowly decrease.

There were another two major flaws in our method:

) We lost the initial burst of oxygen because we were not able to put the bung in the conical flask quick enough.

2) The yeast got stuck in the bubbles of oxygen, which caused the reaction to take place more slowly, because the yeast was not fully in contact with the hydrogen peroxide.

Losing the oxygen was a very big problem because even though it was

only a few seconds the quickest part of the reaction is at the beginning. When you look at our results you can see that all the initial bursts were 4 or 5 cm3. This was because we lost lots of oxygen on the faster reactions so they all seemed to start off at the same rate. The way to stop this would be to use a conical flask like this:

This conical flask is good because you can add all the hydrogen peroxide through the hole in the side using a syringe and a connecting tube.

The bubbles were also a big problem because they stopped the catalase being in contact with the hydrogen peroxide and if the enzyme and the substrate then the reaction wouldn't work. The way to stop this from happening would be to shake the conical flask but this would add another variable and it would be very difficult to shake it the same each time. This would make the results less accurate. We could make it the same each time by putting the conical flask on a vibrating surface.

These improvements would make a vast difference in the accuracy of the results. But there was another major problem to do with the heat of the hydrogen peroxide. As we only heated up the water bath we did not take into account the heat of the H2O2 to begin with. So when we thought we were using 70°C as our temperature, the temperature of the H2O2 was nothing like that high a temperature.

The way to solve this problem would be to heat up both the hydrogen peroxide and the water bath and keep them both at a constant temperature. It was these things that led to some of my results being slightly off what I expected e.g. my 70°C results. I also think that I should have taken 3 sets of results instead of 2, as this would have drawn out any anomalous results more clearly.

If I were to do the experiment again I would use all my improvements to make it a fairer and more accurate test.

Richard McAlpine 10S